Potentially the most advantageous electrochemical couple is the lithium-fluorine couple whose theoretical potential is about 5.9 volts with a theoretical energy density of greater than 5000 Wh/kg. Nevertheless, it is difficult to envisage using fluorine directly because of severe difficulties in handling it. Fluorine is a gas at ambient temperature and under atmospheric pressure and it is extremely reactive.
In contrast, it is possible to envisage using fluorine associated with some other element, thereby obtaining compounds that are more easily handled at a temperature close to ambient temperature. This applies when fluorine is associated with a halogen such as bromine, thereby obtaining a liquid compound such as bromine trifluoride, which liquid can be used as a catholyte in liquid cathode lithium primary cells.
It is observed that lithium and certain aklaline-earth metals are passivated in bromine trifluoride. The excessive passivation of the metals is even somewhat of a limitation on the system. This causes the anode of a lithium and bromine trifluoride cell that has been activated and stored for a few hours in open circuit to cease discharging.
An object of the present invention is to avoid this drawback and to enable a large fraction of the energy of the cell to be recovered even if it has been stored for several tens of hours after being activated.